The present invention relates to spectral imaging and, more particularly, to a method and system for real time high speed high resolution hyper-spectral imaging. The present invention is based on using piezoelectric technology with closed loop control and analysis algorithms, for enabling real time high speed high resolution nanometer accuracy movement of a movable mirror in an optical interferometer, along with using a specially designed and constructed optical interferometer mount as part of the optical interferometer, for achieving high thermo-mechanical stability of mounted optical interferometer components during the real time hyper-spectral imaging. Implementation of the present invention results in high speed collecting of high resolution interferogram images used for synthesizing and analyzing high resolution highly reproducible three-dimensional hyper-spectral (cube) images.
In the general technique of spectral imaging, in most applications, one or more objects in a scene or sample are affected in a way, such as excitation by incident electromagnetic radiation, for example, ultraviolet radiation, supplied by an external source of the electromagnetic radiation, upon the objects, which causes each object to emit electromagnetic radiation in the form of an emission beam featuring an emission spectrum. There are some applications of spectral imaging which don't require an external source of electromagnetic radiation for causing emission by objects, for example, as a result of inherent (body) thermal heat emitted by objects in a scene or sample.
A typical spectral imaging system consists of an automated measurement system and analysis software. The automated measurement system includes optics, mechanics, electronics, and peripheral hardware and software, for irradiating (typically using an illuminating source) a scene or sample, followed by measuring and collecting light emitted (for example, by fluorescence) from objects in the scene or sample, and for applying calibration techniques best suited for extracting desired results from the measurements. Analysis software includes software and mathematical algorithms for analyzing, displaying, and presenting, useful results about the objects in the scene or sample in a meaningful way.
The spectral intensity of each pixel in an optical image of a scene or sample is determined by collecting incident light emitted by objects in the scene or sample, passing the light through an optical interferometer which outputs modulated light corresponding to a set of linear combinations of the spectral intensity of the light emitted from each pixel. Light exiting from the interferometer is focused onto a detector array or matrix, followed by independently and simultaneously scanning the optical path difference (OPD) generated in the interferometer for all pixels, and then processing the outputs of the detector array (a plurality of separate interferograms of all pixels) for determining the spectral intensity of each pixel needed for generating spectral (cube) images. Spectral imaging is practiced by utilizing various different types of interferometers wherein the OPD is varied, in order to synthesize the interferograms, by moving the entire interferometer, by moving an element within the interferometer, or by changing the angle of incidence of the incoming radiation. In each case, optical scanning of the interferometer enables synthesizing interferograms for all pixels of the imaged scene.
Each spectral (cube) image is a three dimensional data set of voxels (volume of pixels) in which two dimensions are spatial coordinates or position, (x, y), in an object and the third dimension is the wavelength, (λ), of the imaged (emitted) light of the object, such that coordinates of each voxel in a spectral (cube) image may be represented as (x, y, λ). Any particular wavelength, (λ), of imaged light of the object is associated with a set of spectral (cube) images each featuring spectral fingerprints of the object in two dimensions, for example, along the x and y directions, whereby voxels having that value of wavelength constitute the pixels of a monochromatic image of the object at that wavelength. Each spectral (cube) image, featuring a range of wavelengths of imaged light of the object is analyzed to produce a two dimensional map of one or more physicochemical properties, for example, geometrical shape, form, or configuration, and dimensions, and/or chemical composition, of the object and/or of components of the object, in a scene or sample.
An example of a spectral imaging technique is that of a method and system for real-time, on-line chemical analysis of particulate samples, for example, polycyclic aromatic hydrocarbon (PAH) particles in aerosols, in which the PAH sample is excited to emit light, for example fluorescence, as disclosed in U.S. Pat. No. 5,880,830, issued to Schechter, and manufactured by GreenVision Systems Ltd. of Tel Aviv, Israel, the disclosure of which is incorporated by reference for all purposes as if fully set forth herein. In the disclosed invention, spectral imaging techniques are used for acquiring images and analyzing properties of fixed position PAH particles in an aerosol. As part of the invention, air is sampled by means of a high volume pump sucking a large volume of air featuring aerosol contaminated with PAH particles onto a substrate, followed by on-line imaging and scene analysis of the stationary particles.
A method of calibration and real-time analysis of particles is described in U.S. Pat. No. 6,091,843, to Moshe et al., and is incorporated by reference for all purposes as if fully set forth herein. The method described, is based on using essentially the same system of U.S. Pat. No. 5,880,830, for acquiring spectral images of static particles on a filter. Targets are identified in static particle images and are classified according to morphology type and spectrum type. Each target is assigned a value of an extensive property. A descriptor vector is formed, where each element of the descriptor vector is the sum of the extensive property values for one target class. The descriptor vector is transformed, for example, to a vector of mass concentrations of chemical species of interest, or of number concentrations of biological species of interest, using a relationship determined in the calibration procedure. In the calibration procedure, spectral images of calibration samples of static particles having known composition are acquired, and empirical morphology types and spectrum types are inferred from the spectral images. Targets are identified in the calibration spectral images, classified according to morphology type and spectrum type, and assigned values of an extensive property. For each calibration sample, a calibration descriptor vector and a calibration concentration vector is formed. A collective relationship between the calibration descriptor vectors and the calibration concentration vectors is found using chemometric methods.
In the method of U.S. Pat. No. 6,091,843, standard spectra are determined empirically in the calibration procedure. In such analytical procedures, empirical calibration is quite important for leading to highly accurate results based on image analysis and classification, because spectra of adsorbed chemical species in general, and, of PAHs in particular, are known to be altered by the surfaces on which they are adsorbed, and by the presence of contaminants during sample preparation and image acquisition. Moreover, in the described method, the relationship between the descriptor vector and the concentration vector accounts explicitly and simultaneously for both morphologies and empirically determined spectra.
In the more specialized technique of ‘hyper-spectral’ imaging, multiple images of each object are generated from object emitted electromagnetic radiation having wavelengths and frequencies associated with different selected parts or ‘bands’ of an entire spectrum emitted by the object. For example, hyper-spectral images of an object are generated from object emitted electromagnetic radiation having wavelengths and frequencies associated with one or more of the following bands of an entire spectrum emitted by the object: the visible band, spanning the wavelength range of about 400-700 nanometers, the infra-red (1R) band, spanning the wavelength range of about 800-1200 nanometers, and the deep infra-red band, spanning the wavelength range of about 3-12 microns. If proper wavelengths and wavelength ranges are used during hyper-spectral imaging, data and information of the hyper-spectral images are optimally used for detecting and analyzing by identifying, discriminating, classifying, and quantifying, the imaged objects and/or materials, for example, by analyzing different signature spectra present in pixels of the hyper-spectral images.
‘High speed’ hyper-spectral imaging system is required for different types of repeatable and non-repeatable chemical and physical processes taking place during the sub-100 millisecond time scale and cannot, therefore, be studied using regular hyper-spectral imaging techniques. Combustion reactions, impulse spectro-electrochemical experiments, and inelastic polymer deformations, are examples of such processes. Remote sensing of objects in distant scenes from rapidly moving platforms, for example, satellites and airplanes, is another example of a quickly changing observable that is often impossible to repeat, and therefore requires high speed hyper-spectral imaging.
There are prior art teachings about hyper-spectral imaging which can be used for obtaining hyper-spectral images of objects in a scene or sample. Specific hardware for hyper-spectral imaging includes filter wheels and circular variable filters as disclosed in U.S. Pat. No. 5,591,981, U.S. Pat. No. 5,784,152, and U.S. Pat. No. 5,410,371; angle-tuned interference filters as in the Renishaw imaging Raman microscope described in U.S. Pat. No. 5,442,438; acousto-optical tunable filters (AOTFs) as disclosed in U.S. Pat. No. 5,216,484, U.S. Pat. No. 5,377,003, U.S. Pat. No. 5,556,790, and U.S. Pat. No. 5,379,065; optical interferometers as disclosed in U.S. Pat. No. 5,835,214, U.S. Pat. No. 5,817,462, U.S. Pat. No. 5,539,517, and U.S. Pat. No. 5,784,162. However, none of these prior art teachings disclose a method or system for enabling high speed grabbing and generating hyper-spectral (cube) images, self spatial and spectral calibration capabilities and procedures, and rapid tunable fast scanning rate of, for example, less than 50 millisecond for an entire spectral (cube) image, as required by more sophisticated applications of hyper-spectral imaging.
Currently available hyper-spectral imaging techniques are significantly limited by having insufficiently high speeds of generating and processing spectral (cube) images, and are limited by providing insufficiently high resolution of the spectral (cube) images, as a result of low thermo-mechanical stability of hyper-spectral imaging system components, for example, mounted components such as beam splitters and mirrors in an optical interferometer, along with inaccuracy in measuring the optical path difference (OPD) of a divided object emission beam. Additionally, current hyper-spectral imaging techniques are significantly limited when employed in the wavelength range of about 100 nm to about 800 nm. In this spectral range, during the generating and collecting of spectral data using prior art hyper-spectral imaging techniques, typically, spatial errors and spectral errors are intrinsically generated and translate directly to decreasing quality (resolution) and reproducibility of the hyper-spectral images of objects in a scene or sample.
There is thus a need for, and it would be highly advantageous to have a method and system for real time high speed high resolution hyper-spectral imaging. Moreover, there is a need for such an invention which is based on using piezoelectric technology with closed loop control and analysis algorithms, for enabling real time high speed high resolution nanometer accuracy movement of a movable mirror in an optical interferometer, along with using a specially designed and constructed optical interferometer mount as part of the optical interferometer, for achieving high thermo-mechanical stability of mounted optical interferometer components during the real time hyper-spectral imaging, resulting in high speed collecting of high resolution interferogram images used for synthesizing and analyzing high resolution highly reproducible three-dimensional hyper-spectral (cube) images.